Recently, in optical communication systems, for example, optical modulators employing semiconductors such as indium phosphorus (InP) (hereinafter referred to as the “semiconductor optical modulator”) have been under development in place of optical modulators employing lithium niobate (LiNbO3) (hereinafter referred to as the “LN optical modulator”). When compared with the LN modulator, the semiconductor modulator can attain a higher efficiency of applying an electric field, thus facilitating reduction in drive voltage and size. That is, when compared with LN optical modulator, the semiconductor optical modulator can be more easily reduced in size.
However, the semiconductor waveguide confines light at a higher level when compared with the lithium niobate (LiNbO3) waveguide, with the waveguide mode profile significantly reduced. Thus, when a signal beam outputted from the semiconductor optical modulator is emitted from the optical waveguide toward a downstream optical fiber, the angle of divergence of the signal beam is greater than the angle of divergence of a signal beam outputted from the LN optical modulator. The increase in the angle of divergence of the signal beam is not preferable due to the possibility of causing interference between signal beams.
In this context, such a technique has been suggested by which a collimate lens that collimates the signal beam is disposed between the optical waveguide and a condenser lens that focuses the signal beam on the optical fiber. This technique allows the collimate lens to collimate the signal beam and thereby reduce an increase in the angle of divergence of the signal beam, so that the interference of signal beams is prevented.
Patent Document 1: Japanese Laid-open Patent Publication No. 2007-201939 is introduced as the Prior Art Document.
However, it is difficult to improve the coupling efficiency between the optical waveguide and the optical fiber only by interposing the collimate lens between the optical waveguide and the condenser lens as in the aforementioned conventional technique.
That is, even when the increase in the angle of divergence of the signal beam is reduced, it is known in an optical transmitter with the semiconductor optical modulator that the diameter of a vertical cross section of a signal beam along the direction of the thickness of a substrate on which the optical waveguide is formed is greater than the diameter of a horizontal cross section of the signal beam along the direction of the width of the substrate. In other words, for the optical transmitter with the semiconductor optical modulator, a cross-sectional shape of the signal beam when viewed in the direction of travel of the signal beam emitted from the optical waveguide toward the optical fiber takes an elliptical shape with the major axis along the direction of the thickness of the substrate and the minor axis along the direction of the width of the substrate. Thus, the cross-sectional shape of the signal beam focused by the condenser lens on the optical fiber is also maintained in an elliptical shape. This may possibly cause a loss in the light by allowing the signal beam having an elliptical cross-sectional shape to be incident on the optical fiber having a circular cross-sectional shape. That is, when the semiconductor optical modulator is employed, the cross-sectional shape of the signal beam emitted from the optical waveguide does not coincide with the cross-sectional shape of the optical fiber, thus possibly causing the coupling efficiency between the optical waveguide and the optical fiber to be lowered.
In response to this, it can be conceived to provide such a structure in which a prism is additionally disposed between the collimate lens and the condenser lens in order to form the cross-sectional shape of the signal beam emitted from the optical waveguide into the cross-sectional shape of the optical fiber.
However, the structure in which the prism is additionally disposed leads to an increase in the parts count by the prism, thus possibly causing increases in the complexity of the structure and the size of the apparatus.